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Stop, look, listen: high resolution cameras and ultrasonics have developed rapidly but are still no substitute for skilled human inspectors.

Automated inspection of welds is the holy grail of the industry, but it remains elusive. Critical welds are still best inspected by the Mk1 Eyeball, according to many experts. Technology has progressed in leaps and bounds over the past few years, but both visual and ultrasonic inspection systems have drawbacks that limits their use to non-critical joins, says Malcolm Spicer, NDT consultant Level 3 at The Welding Institute (TWT).

"Just about every weld except a quick tack weld should be inspected visually," he says. "The visual welding inspector should be properly qualified, especially if the joint is critical, such as a joint in a subsea pipeline that needs to withstand a lot of pressure underwater for many years."

Despite rapid progress in both visual and ultrasonic inspection technologies in the past few years, human beings remain an essential part of the inspection process.

Ultrasound systems have progressed by leaps and bounds over recent years, Spicer says, but still have limitations.

"Ultrasound gives a high level of assurance by looking for internal defects that the visual inspector can't see, such as lack of fusion, inclusions, porosity and cracks," he says. "The medical field has had this for years for foetal monitoring and now we are getting to the stage where we can apply it to industrial components."

Phased arrays have transformed ultrasonics, making it an essential tool for the inspector. "The big advance is phased arrays where an array of piezoelectric crystal is fired in phase to create a beam that can be steered through the test piece. We can carry out the inspection a lot more we quickly and gain a lot more information about the internal structure of the component" Spicer says. "The beam can be steered between 35 and 70 degrees and the image displayed in glorious technicolour on the screen."

The next step, Spicer says, is full matrix capture in which a complex computer-based system of transducers floods the area with sound, listening to the returns to build up an internal image of the component. "It is heading towards the point where we can produce a 3D image of the component like a CT scan slicing the brain," says Spicer. "It still needs expert interpretation but there is a lot of information there thanks to computer technology."

A major advantage of a digital system is, of course, that all the data can be stored easily and cheaply so it can be retrieved and displayed for re-analysis in the future, something that was often impractical in analogue systems.

But Spicer warns against relying too much on computer technology. "Bear in mind that both methods are good at detecting some types of flaw but miss others. Most applications still need a visual inspection for any signs the weld was not carried out correctly."

The latest digital imaging systems linking cameras to computers running software to analyse the images automatically still have limitations. "They do have their uses but bring other problems of lighting and resolution and getting the camera in the right position," says Spicer. "Automatic recognition systems have limited use, I think, so we tend to think of visual inspection as having a human being involved - automatic defect recognition systems are still not as good as human beings."

Even visual systems that simply provide cameras to take very high resolution images for the inspector to view have to be treated with care, Spicer believes.

"Assisted vision is getting better and better as cameras with very high resolution arrive, but the danger with enhanced resolution is that you may enlarge something that's benign and mistake it for something harmful."

Torn Ward, product manager for visual inspection systems at GE in the US, believes the main advantage of cameras is the ability to take inspectors to places they cannot conveniently visit personally

"Remote visual inspection equipment, like video borescopes and remote cameras, can squeeze into extremely tight spaces such as inside engines or pipelines, and can also be used to access confined spaces or hazardous environments where special access measures, atmospheric purging and permits could be required for human inspectors," says Ward. "Video borescopes can save inspectors time by allowing inspection in-situ rather than disassembling equipment, and by eliminating the need for confined space entry in some situations."

The image output from cameras is dramatically improving not only because the cameras have higher resolution, adaptive noise reduction and high dynamic range, but advances in LED technology illuminate the subject better, Ward says.

However, Ward and Spicer agree that the inspector is not going to disappear any time soon. In fact, Ward believes the technology will allow many inspectors to examine critical welds: "There is no substitute for human judgment, but advancements in connectivity are helping put more eyes on inspections remotely."

Caption: Picture perfect: Digital cameras take enhanced images of welds such as this one taken by a GE Mentor Visual iQ VideoProbe
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Title Annotation:Inspection
Author:Partridge, Chris
Publication:Environmental Engineering
Date:Oct 1, 2014
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